The present invention relates to a gasket which, intervening between the joining faces of a cylinder block and a cylinder head, is caused to seal the gap between the joining faces by being fastened with fastening bolts.
Gaskets each having a base plate and a shim plate include ones disclosed in JP2002-5292A (hereinafter referred to as Patent Document 1) and JP2002-31237A (hereinafter referred to as Patent Document 2), for instance.
In the gasket disclosed in Patent Document 1, a base plate bead is disposed around a combustion chamber hole in a base plate, and a shim plate is so arranged opposite the base plate as to cover that base plate bead. That shim plate is large enough to oppose the base plate from the edge of the combustion chamber hole in the base plate to the position of a water hole farther outside then the base plate bead. On the shim plate, only one strip of auxiliary bead is so formed as to come into contact with a flat part of the base plate from the bead toward the combustion chamber and a flat part farther out than the combustion chamber. Intervening between the joining faces of the cylinder block and of the cylinder head, the gasket is intended, even if a gap is caused to arise in any less rigid part by the deformation of the engine or any other cause when fastening bolts are fastened, to prevent gas or cooling water from flowing into the base plate bead side as the auxiliary bead formed on the shim plate follows the deformation.
Also in the gasket disclosed in Patent Document 2, a base plate bead is disposed around a combustion chamber hole in a base plate, and a shim plate is so arranged opposite the base plate as to cover that base plate bead. That shim plate is large enough to oppose the base plate from the edge of the combustion chamber hole in the base plate to the position of a water hole farther outside then the base plate bead. The end part of the shim plate toward the combustion chamber hole is folded back to have a double structure, and an auxiliary bead is formed farther out than the base plate bead. Intervening between the joining faces of the cylinder block and of the cylinder head, the gasket deforms, when the fastening bolts are fastened, the engine as much as the difference in plate thickness in the gasket to make the contact pressure higher on the combustion chamber hole than on the base plate bead.
However, since the engine is not a rigid body, a difference in deformation arises between the fastening bolts. To improve this difference in deformation, the doubled side end part of the combustion chamber in the shim plate where the contact pressure is higher is so formed by forging as to vary in thickness in the circumferential direction, so as to uniformize the contact pressure in the circumferential direction by lowering the contact pressure where it is high and raising the contact pressure where it is low.
As an aspect of the rapid progress of technological innovation in recent years, engines are increasingly reduced in size and weight, and this tends to entail a further decrease in engine rigidity. In the gasket according to Patent Document 1, since the shim plate disposed along with the base plate is thinner than the base plate, the spring force of the auxiliary bead formed on the shim plate is correspondingly weaker. Also, as there is only one strip of auxiliary bead formed closer to the combustion chamber than the base plate bead, the contact pressure generated by that auxiliary bead is also lower. As a result, especially between bolts under low contact pressure, the high combustion pressure may deform the single-strip auxiliary bead of the shim plate to let high-temperature gas pass the auxiliary bead to reach the base plate bead. The present inventors found that this heat could also have long-term adverse effects including gradual deterioration of the base plate bead.
In the gasket disclosed in Patent Document 2 on the other hand, the end part of the shim plate toward the combustion chamber hole is folded back to have a double structure with a view to increasing the contact pressure. At the same time, it is also intended to increase the contact pressure in the vicinities of the combustion chamber and uniformize the pressure in the circumferential direction. Though this proves more or less effective in a stationary state, its application to modern engines, which are further weakened in rigidity and made more susceptible to thermal deformation due to a rise in combustion temperature as a result of reductions in size and weight and enhancement in performance entailing increases in the number of valves and the compression ratio and involving improvement in diluted fuel combustion techniques, all as aspects of the rapid progress of technological innovation in recent years. Especially, the application of such gaskets to engines which are subjected to greater thermal deformation between bores due to repetition of rapid heating from a cold state and then of cooling, even if the technique described in Patent Document 1 is used in combination with that of Patent Document 2, would invite pulsation of cylinder heads caused by the explosive pressure of combustion occurring simultaneously with the thermal deformation. Although the deformation could be absorbed in the vicinities of bolts by the elastic region of the bolts, the compliance with deformation would prove insufficient between bolts and between bores for its absorption, and this could let the gas blow through. These problems have also been identified by the inventors, and they would arise more frequently when such gaskets are applied to engines of a high compression ratio, especially diesel engines.
An object of the present invention, attempted in view of these problems, is to provide a gasket with a sufficient sealing capability even for compact, light, less rigid and powerful engines.